This invention relates broadly to thermoplastic formulations, particularly formulations having a low glass transition point. In one aspect this invention relates to a liner for high explosive devices. In another aspect, this invention relates to thermoplastic formulations.
General Purpose (GP) aerial bombs are lined with a thermoplastic material prior to filling with an explosive in order to provide a barrier between the explosive and the bomb case. The practice of lining GP bombs began in the late 1950's in order to provide an impact cushion for the weapons, particularly in areas of possible pinch points such as the nose and tail fusewells and flange assembly threads. As ammonium nitrate (AN) based explosives came into general use, the liner assumed importance as a mechanical barrier to prevent chemical interaction between the explosive and the case.
The range of qualities required or desired of a bomb liner includes the following:
a. chemical compatibility with the explosive and with the bomb case, in order to prevent decomposition of the explosive, gas generation and or sensitization during storage;
b. low or nil permeability to moisture in order to prevent ingress of moisture which could accelerate decomposition of the explosive, particularly AN based explosives;
c. good adhesion to the bomb case;
d. resilience when subjected to rapid temperature cycling in order to maintain liner integrity;
e. dimensional stability against cold flow during storage;
f. resilience when subjected to impact such as when the bomb is dropped a few feet;
g. ease of installation of the liner together with the ability to withstand the heat of a melt cast explosive at about 220.degree. F.; and
h. relatively low cost.
Roof asphalt was initially adopted as the primary bomb liner because of its low cost, ease of application and availability. The asphalt is molten at 400.degree. F. and can be applied by a simple pour in/pour out technique. It can withstand the heat of a melt cast explosive at about 220.degree. F. It has excellent low temperature resilience, having an operational temperature range of -65.degree. F. to +160.degree. F., with an expected operational lifetime of 20 years.
Asphalt is not a pure, homogeneous product. Depending upon its source, asphalt can contain up to 6% or more of sulfur and up to 25% or more of mineral matter. The sulfur and mineral matter in asphalt are generally incompatible with explosives. Compatibility problems have been noted in asphalt-lined bombs containing ammonium nitrate-based explosives, in particular, as well as in bombs containing other explosives.
It has been proposed to replace the asphalt bomb liner with amorphpous polypropylene. Although it is chemically compatible with the high explosives, it is not fully satisfactory as a replacement liner, primarily because of its physical behavior.
Accordingly, it is an object of the present invention to provide a novel thermoplastic formulation which satisfies the requirements for a bomb liner.
Another object of the present invention is to provide a general purpose aerial bomb having an improved thermoplastic liner.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from the following description of the invention.